I was using them on the flat bench for six reps. I would do a five second negative and could only maintain that control for the first four reps. It felt like the bar was starting to fall by the fifth and sixth rep. My training partner had to do a rack pull to pull the bar off of me as I was so weak.
I would do that for two sets at the end of my chest workout. Now that I think about what you wrote, I was most likely doing too much.
I think they would be good to do right before a long vacation.